1. Field of Invention
The present invention relates generally to the art of flash based solid state storage.
2. Description of Related Art
RAID (Redundant Array of Independent Disks) is a storage system used to increase performance and provide fault tolerance. RAID is a set of two or more hard disks and a specialized disk controller that contains the RAID functionality. RAID improves performance by disk striping, which interleaves bytes or groups of bytes across multiple drives, so more than one disk is reading and writing simultaneously (e.g., RAID 0). Fault tolerance is achieved by mirroring or parity. Mirroring is 100% duplication of the data on two drives (e.g., RAID 1).
A volume in RAID storage system is a virtual disk drive. The volume appears as a disk drive for computer systems attached to the RAID storage systems. Volumes in RAID systems may span several physical hard drives, may be entirely contained as a portion of a single physical drive, or may be mapped across other virtual drives in the RAID system. An enclosure is a physical structure housing one or more physical disk drives. The enclosure may also contain a RAID controller or may contain storage connection hardware to enable attached computers and RAID controllers to communicate to the physical hard drives housed in the enclosure. A Logical Unit (LU) is a SCSI term for an addressable entity within a SCSI peripheral device. Volumes in RAID storage systems that are externally addressable via the SCSI protocol are typically addressed as Logical Units and assigned a Logical Unit Number (LUN) designator. Volumes in a RAID-system are therefore sometimes referred to as LUNs.
A Storage Area Network (SAN) often connects multiple servers to a multiple storage devices and storage systems. In some SANs, the storage devices themselves can copy data to other storage devices for backup without any computer processing overhead. The SAN network allows data transfers between computers and storage systems at high peripheral channel speeds.
A host adapter, also called a “controller” or “host bus adapter,” it is a device that connects one or more peripheral units to a computer. The host adapter can also connect the computer to a SAN. It is typically an expansion card that plugs into the bus. SCSI, SAS, Fibre Channel, iSCSI and Infiniband are examples of peripheral interfaces that call their controllers host adapters. A host can be a computer that runs an application and accesses storage systems and devices directly attached to the computer or attached over a SAN.
Flash memory (both NAND and NOR types) is non-volatile memory. One limitation of flash memory is that although it can be read or programmed a byte or a word (NOR) at a time or at a page (NAND) at a time in a random access fashion, it must be erased a “block” at a time. Flash memory (specifically NOR flash) offers random-access read and programming operations, but cannot offer arbitrary random-access rewrite or erase operations. Another limitation is that flash memory has a finite number of erase-write cycles. Most commercially available flash products, in particular NOR types, are guaranteed to withstand around 100,000 write-erase-cycles. NAND flash comes in two types: single level cell (SLC) and multiple level cell (MLC). SLC NAND flash stores one bit per cell while MLC NAND flash can store more than one bit per cell. SLC NAND flash has write endurance equivalent to NOR flash, 100,000 write-erase cycles, while MLC clash write endurance is 10,000 write erase cycles or less. NAND Flash is Less expensive than NOR, and erasing and writing NAND is faster than NOR.
The NAND flash page is the smallest unit of memory which can be written or read, a typical size is 2048 bytes. The smallest erasable unit of flash memory is a block. A typical block contains 64 pages or 128K Bytes. NAND flash architecture was introduced by Toshiba in 1989. These memories are accessed much like block devices such as hard disks or memory cards. Each block consists of a number of pages. The pages are typically 512 or 2,048 or 4,096 bytes in size. Associated with each page are a few bytes (typically 12-16 bytes) that should be used for storage of an error detection and correction checksum. Typical block sizes include: 32 pages of 512 bytes each for a block size of 16 kiB; 64 pages of 2,048 bytes each for a block size of 128 kiB; 64 pages of 4,096 bytes each for a block size of 256 kiB; 128 pages of 4,096 bytes each for a block size of 512 kiB. While reading and programming is performed on a page basis, erasure can only be performed on a block basis.
A solid state disk or device (SSD) is a device that uses solid state technology to store its information and provides access to the stored information via a storage interface. SSDs are faster than hard disk drives using spinning platters because there is no mechanical latency, as there is no read/write head to move and no spinning disk to wait for, as in a traditional drive. SSDs are more rugged than hard disks. SSDs may use non-volatile flash memory; or, SSDs may use volatile DRAM or SRAM memory backed up by a disk drive or UPS system in case of power failure, all of which are part of the SSD system.
Traditional storage systems can execute a point-in-time copy of data in storage systems and subsystems, termed a time copy or snapshot. One purpose of the snapshot is to allow a backup operation to run while the application continues to use the current copy of data. Another use of the snapshot is to allow data mining on a copy of the data without impacting the ongoing use of the active data set. Techniques to do snapshot are “copy on write” and “redirect on write”, as explained further herein.
Flash based SSDs place data on their internal physical devices according to algorithms which optimize data placement for parameters such as performance and wear leveling. These SSD maintain mapping or lookup tables that relate and translate data addresses between their logical drives or units and the physical devices within the SSD. Like RAID systems, SSDs maintain mapping or lookup tables that relate and translate data addresses between their logical drives or units and the physical devices within the SSD. In an SSD, the data in non-volatile memory is segmented into pages, with typically for a 128K block of data there exist a plurality of data segments, each holding about 2K of data. The correspondence between data (e.g., a data segment), pages, and flash blocks (an area of non-volatile memory, X, Y, Z etc) is maintained in one or more mapping tables, which may be used by hardware to lookup the correct address of data. However, in contrast to the present invention, these mapping tables only point to the current copy of the data, which is not of use for creating snapshots.
What is lacking in the prior art is a method and apparatus for an improved system to perform a time copy or snapshot for solid state devices (SSD), such as taught in the present invention.